Although uninstalling Adobe Reader and rebooting it help at the first instance, but then there are times, when this do not work.
1. Click on any other icon on desktop, other than Adobe Reader, which is present. Right click "open with" and select the correct programm to open it with. Select it and always use this as default for this kind. Apply & Run. Voila, all done.
2. In case this hampers anything, you can switch to the restore point of an earlier time. The latest is always disabled to be used by the system. Restart and done.
3. If none, then uninstall the app, restart, change as per point 1, and restart. If you require reader, install it but with advanced modes on & change, as not default.